Ethylene and propylene are important building blocks for the petrochemical industry, and find use for production of polymers such as polyethylene, polypropylene, polystyrene and many others. Additionally, ethylene and propylene are used in the production of many other chemicals of commercial interest.
While a number of chemical processes are known for forming ethylene and propylene, the most commonly used process is thermal dehydrogenation of gaseous hydrocarbons or high-temperature steam cracking of light liquid components derived from processing of crude oil and natural gas condensate (e.g. ethane). In this process, gases such as ethane and propane are exposed to very high temperature conditions, resulting in the stripping of a hydrogen atom from each of two adjacent carbon atoms, forming the corresponding olefin. However, the process is very endothermic, and a great deal of energy must be generated to elevate the temperature of both the hydrocarbon and the reactor to a level sufficient to conduct the dehydrogenation.
Another method of dehydrogenation of hydrocarbons which has been proposed is oxidative dehydrogenation (ODH), in which an oxygen transfer agent (OTA) or oxidation catalyst is contacted with, for example, ethane under moderate temperature conditions, and one hydrogen from each carbon combines with an oxygen atom of the oxygen transfer agent or catalyst, to provide ethylene and water as the main products. An advantage of ODH is that less H2 and CH4 are formed during the reaction as compared to the amounts of those byproducts produced during thermal dehydrogenation processes. One disadvantage of the ODH reaction is that carbon monoxide (CO) and carbon dioxide (CO2) are formed as low-value byproducts. A further disadvantage of ODH reactions is that nitrogen must be separated from either the feed air or the reactor product. Nitrogen separation from oxygen or light olefins by distillation is expensive because of the low boiling points of these species.
Chemical looping (CL) is a dynamic process in which a material, e.g., a metal oxide (which acts as an oxygen transfer agent or OTA) is used to provide an element such as oxygen for a reaction during which the material itself undergoes reduction. The reduced oxide or metal is then reoxidized in either a second reactor or in a second step if a fixed bed is used. The result is that a CL process physically (or temporally) separates an overall process into its separate oxidation and reduction steps through the use of a solid phase material capable of itself undergoing oxidation and reduction. CL has previously been applied to energy conversion, reforming, and water-gas shift processes. Importantly, the oxidizing and reducing streams fed to a CL process are never mixed with each other, and thus, an ‘unmixed’ reaction is performed.
The CL approach enables an ODH reactor that overcomes the challenge of nitrogen separation by distillation. In one form, at least two fixed bed reactors are used, each having a bed of OTA, at least a first reactor having the OTA in the oxidized form and at least a second reactor having the OTA in a reduced form. The ODH is conducted in the first reactor having the oxidized OTA bed, while the reduced OTA bed in the second reactor is regenerated with an oxygen-containing gas. Once the OTA in the first reactor is exhausted, the ODH reaction is switched to the second reactor and the first is subjected to regeneration/oxidation. The reactors are heat balanced such that the heat released by oxidizing the OTA supplies the energy needed for the process. In some cases, an extra source of energy is used to heat the feeds to the reactor, which adds cost and undesirable emissions.
Other reactors suggested for the process include moving bed reactors, such as fluidized bed reactors, which are quite expensive and complex in operation. A disadvantage of fixed and moving bed reactors is that they have limited control over the gas temperature, which is important for reactions. In both cases, a cooler hydrocarbon feed must contact a hot OTA to initiate the reaction. Therefore, the hydrocarbon may contact the OTA before thermal cracking reactions occur, which may affect and even depress reaction yield. The flows of regeneration and hydrocarbon gases are in the same direction, meaning the effluents exit the reactor at about the maximum temperature of the vapors in the reactor. This design increases the heat demand of the process and the capital employed for heat exchange. In addition, it is challenging to design a fast quench system for thermal reactions in such reactors, in part because the heat exchanger is typically located outside the reactor. A fast quench is typically needed to achieve high yields in such reactions. Furthermore, the fast quench is typically achieved by transferring heat from the olefin product to boil water in a heat exchanger after the reactor, which requires extra energy to heat up and boil water. It would be advantageous to find a suitable flow-through, fixed bed reactor with which to conduct chemical looping ODH, and effective OTA materials for such use.
Some materials suggested for use as OTA materials include Mn/B/MgO, Li/Mn/B/MgO, P/W/Li/Mn/B/MgO, and Na/B/Mn/Mg, and Mn/Na/P/SiO2. However, these materials are not optimum as OTA materials for ODH reactions because they may convert hydrocarbons to products comprising an undesirably high yield of CO2.
Some other materials suggested for the OTA include CaxLa1-xMn1-yMyO3-n wherein M is an element selected from the group consisting of Mg, Ti, Fe, and Cu. Another material includes CaMnO3 doped with La, Fe, Sr, or Zr. These materials converted hydrocarbons to a product comprising an undesirably high yield of CO2.